Strange sound when starting engine

[OOge]

I've noticed that my ABS and TCS lights comes up once in a while. But today I noticed that when starting the engine and pressing the brake pedal, a strange noise is coming from the power brake unit area when releasing the brake pedal.

So I started the engine without pressing the brake pedal. The same sound appears. I can't explain the sound, but it sounds like a cylinder is moving without oil. When starting the engine again the sound comes back at the release of the brake pedal.
Any one an idea where to look. Is it the power brake unit ?
The sound seems to come from drivers side under the hood in the power brake unit area.